These naming conventions are rooted in the world of object orientated coding, an object can inherit from a base object. So the base object is a layer and this will have properties that all other layer types will inherit, an obvious one is the name.

FeatureLayers and RasterLayers have many other data specific properties, but both of these inherit from Layer.

So Layer is not an abbreviation of FeatureLayer it is a base object (you could think of it as a simpler object) that other types of layers are inheriting from.